The researchers from the universities of South California and Hawaii, found that women who ate one quarter of a grapefruit or more every day had a higher risk of breast cancer than those who did not eat the fruit at all.

The researchers suspect that the fruit boosts the levels of oestrogen in the body and the hormone has previously been linked with a higher risk of the disease.

A study of 50,000 post-menopausal women from five ethnic groups, including 1,657 with breast cancer, found those who ate a quarter of a grapefruit daily raised the risk by up to 30%.

But the researchers along with other experts say more research is needed, but say the study does provide interesting information which will eventually lead to a better understanding of how diet affects health.

For the study the women completed questionnaires on how often they ate grapefruit and how big the fruit was and found in women who ate at least a quarter of a grapefruit daily, levels of oestrogen were higher.

Other studies have shown that a molecule called cytochrome P450 3A4 (CYP3A4) is involved in metabolising oestrogen hormones and it is thought that grapefruit may boost blood oestrogen levels by inhibiting this molecule, allowing the hormones to build up.

The research team say it is well established that oestrogen is associated with a breast cancer risk so if grapefruit intake affects oestrogen metabolism leading to higher circulating levels, then it is biologically plausible that regular intake of grapefruit would increase a woman's risk of breast cancer.

The researchers say this is the first time a commonly eaten food had been linked to an increased risk of breast cancer in older women but warn that more research is needed to confirm the findings because the research only took into account intake of the fruit, but not grapefruit juice.

Breast cancer accounts for almost a third of all cancers in women, and previous lifestyle factors linked to the disease include drinking alcohol and being overweight.

A major European study also recently showed that eating junk food raises women's risk of developing a range of cancers.

The study is published in the British Journal of Cancer.

Grapefruit is known to boost absorption of certain drugs and Professor Ratain says drinking grapefruit juice, which is known to increase the rate at which some drugs enter the blood stream could increase these savings even further.

Ratain also suggests that eating such "value meals" at the same time as taking drugs could have other benefits too; a major toxicity associated with lapatinib is diarrhoea, which is thought to be caused by some of the drug being unabsorbed, therefore taking lower doses with food to boost its absorption should help reduce this side-effects.

He says that one 250mg lapatinib pill accompanied by food and a glass of grapefruit juice may yield plasma concentrations comparable to five pills on an empty stomach.

The authors caution against patients experimenting themselves and say more research to assess the effects of drug-combinations on patients.

They are in fact currently conducting a study testing the effect of combining a drug with grapefruit juice.

The Medicines and Healthcare Products Regulatory Agency says any new recommendations about drug dosages need to be backed up by evidence and full testing through clinical trials.

The research is published in the Journal of Clinical Oncology.